The Green Baize Table Conspiracy

The insanely overdue writeup of my review talk, Quantum entanglement at the psi(3770) and Upsilon(4S), is now on the arXiv public preprint server, and will be included in the proceedings of the Flavor Physics and CP Violation conference. I was asked to give the review because of my involvement in a test of quantum mechanics by the Belle experiment: I wrote about this last year in the post Tangled up in (quantum) blue.

When I have spoken on these results, the most popular part of the talk has always been my explanation (included in the writeup) of an important counter-example, in the form of a conspiracy theory involving the Cigarette-Smoking Man. It seems that more physicists watch the X-Files than would generally admit to it.

The burden of the counter-example is that, if one is willing to countenance bizarre and conspiratorial alternative theories, the sort of study we did at Belle doesn't establish the quantum mechanical result it's trying to test. So we just do the best we can. However there are optical experiments that are immune to (at least this type of) conspiracy-theory explanation. This is the really remarkable thing, and why Bell inequality tests are considered such a big deal in physics. Based on such experiments, we conclude that the weirdness of quantum mechanics is a real feature of the world --- independent of whether quantum mechanics itself is ultimately correct. We might one day learn more, and go beyond the understanding we have from QM, but even so we'd be stuck with the fact that two objects "separated" in space, even many kilometres apart, can be (in a sense) inseparable, forming a single object, a single system. A whole, rather than two parts.

Mongolia via Taiwan, with assistance from Iran and Poland

I've recently been in Taipei for the Flavor Physics and CP Violation conference, the highlight of which was the ominously named "cultural activity" ... a trip to a local auditorium to hear the Mongolian singer Urna perform together with the Chemirani Trio on zarb drums (and other percussion), and the wonderful Jerzy Bawol. (I will never say a bad word about the accordian again, I swear it.)

This was splendidly accessible, serious, light-hearted, joyful music, without a trace of irony. Terrific stuff. There are samples on the Urna website I linked, but sadly none from the particular collaboration that I saw on the 6th.

It's somehow appropriate that one could find so impeccably international a collaboration in a place that does not even belong to the United Nations ...

[The slides of my presentation at FPCP, a review of "Quantum entanglement at the ψ(3770) and Υ(4S)", can be downloaded from the conference site. Regular readers of this blog may recognise the principal result, which was previously remarked under "Tangled up in (quantum) blue". Particle physicists (and some other physicists) should have no trouble with the slides, but I guess they'll be somewhat heavy going for anyone else. The writeup for the conference proceedings will, I hope, be a bit more accessible. I will link it here when it's done.]

UPDATE (13th Oct 2008): The writeup of my talk is now available on the arXiv server. I discuss it in a new post on this blog.

Tangled up in (quantum) blue

An object goes "bang" in the middle of a room and two pieces go flying off, one to the left and one to the right. Each one follows the laws of physics just by itself, and is influenced only by things that touch it (and gravity etc.^[1]). To know what the left piece will do, you needn't bother about the right piece in any special way: no more than you'd bother about the influence of your shoes, the ceiling, or the planet Jupiter.

Sorry: not true, so far as we understand.

If you believe quantum mechanics, those two pieces are in a certain sense still a single object ... even if the two pieces are on opposite sides of the room, or in separate towns many kilometres apart. It's called entanglement, a.k.a. "spooky action at a distance", a.k.a. "weird quantum s***".

I mention this because our paper on quantum entanglement at the Belle experiment has been published in Physical Review Letters.^[2] The theory does just fine at predicting our data --- that's not unexpected, since it's done just fine on all the data it's been confronted with. The real interest in this kind of measurement is to see if one can go beyond testing quantum predictions, and test entanglement itself: to show that entanglement is just-a-fact-about-how-the-world-is-put-together which we'll always be stuck with, even if we eventually improve on quantum mechanics in some way.^[3]

The gold standard for proving entanglement is a theorem by the late John Bell (no relation): our experiment couldn't meet this standard, even if our equipment were perfect (for rather technical reasons). What we can do is put other specific models --- other ways of explaining the data that don't involve entanglement --- to the test. The ones we have been able to try, fail; quantum mechanics succeeds. So entanglement wins this round, yet again, but some alternatives still live to fight another day ...

Here ends the lesson. It's not usually my aim to post such pedagogical material on this site, but there is no end to the flaky silliness on these topics doing the rounds in popular culture, so I feel some kind of duty to fly the flag when I've been a part of the work. What the bleep do we know? Um, well, quite a bit actually.

^[1]Gravitational and electromagnetic forces act "at a distance" but their influence is not instantaneous: it's bound by the speed of light. For everyday purposes that's so fast that the influence might as well be instantaneous, but a lot hangs on the distinction. You can think of it this way: it's the gravitational and electromagnetic fields right where you are ("touching" you) that affect you, and they take time to catch up on what's going on elsewhere, the same as you do. These forces are still local in this sense.
The "spooky" part about quantum entanglement is that the connection between the parts of an entangled system works without any regard to distance whatsoever --- with no speed limit --- yet it turns out that you still can't use the thing to send a signal faster than the speed of light. Put like that, it seems somewhat contrived, and this is one of the things behind the intuition that it's our assumption of separability that's the problem, not the assumption of locality: it's not that relativity doesn't describe spacetime, it's that things really can't be divided up into "parts" the way we tend to think they can.

^[2] It's also publicly available on the arXiv preprint server as quant-ph/0702267.

^[3] Like many people (physicists included) I have my doubts about quantum mechanics: I suspect that there's something more going on. However, I also suspect that the "something more" will still leave us stuck with entanglement: that the weirdness is real.